This application claims the benefit of filing date of Australian Patent Application No. 58291/99, filed Nov. 4, 1999, now Patent No. 719,185.
The present invention is in the field of tire retreading apparatus, and relates to improvements in both the apparatus and method used for retreading tires. In particular, the invention relates to rasp blades, and an improved assembly and method of use of such blades on tire rasp hubs for buffing away the tread on worn tires.
The conventional process by which tire casings are retreaded is to buff away the worn tread, repair any damage that may be required in the casing after buffing, bond a new tread to the casing by a selected vulcanizing process, and then cure the rubber so as to harden and shape it into the desired tread design.
In order to buff and remove the worn tread, the casing is mounted on a buffing machine and inflated. A tire buffing rasp hub, comprising a hub core having a large number of toothed rasp blades mounted thereon, is then rapidly rotated, for example, on a motor driven shaft, and the peripheral surface of the casing bearing the worn tread is forced against the rotating rasp hub to loosen, tear and grind off the excess rubber and roughen the remaining surface sufficiently so that the buffed surface of the casing can form a sufficient bond with the new replacement rubber tread in the vulcanizing process. Each tire size has a predetermined crown width, profile and radius and the casing must be buffed to the particular shape, size and texture to receive a new tread that ensures proper tread-to-road contact. Buffing of the worn tread is, therefore, a critically important stage of the retreading process affecting the quality, performance and safety of retreaded tires.
Rasp blades of the prior art comprise numerous configurations and shapes, a preferred type having teeth of essentially dove-tail shape projecting from the outer working edge. Individual teeth have a notch cut out from the center of the periphery or working edge of the tooth to form a series of substantially xe2x80x9cYxe2x80x9d shaped teeth defined by cutouts of partly circular shape. The notch formed in each tooth divides it into halves. Each half of a given tooth is offset to opposite sides of the general plane in which the blade lies, thus creating a primary cutting edge followed by a laterally spaced buffing edge for each tooth. As the rasp hub is rotated, the primary cutting edge and the buffing edge prepare the surface of the casing to a texture necessary to bond new rubber to it.
A typical tire buffing rasp hub which includes such rasp blades as aforementioned has the form of a hub core defined by interconnected front and back cylindrical end plates having mounted therebetween arcuate or quadrant-shaped rasp blades stacked in four (or more) separate arrays around the perimeter of the core. Each rasp blade of any one stack is separated from adjacent blades of the stack by spacers, the stack being secured in position between the end plates by support pins. Removable fasteners hold the end plates together, sandwiching the rasp blades, thus allowing for dismantling of the rasp hub for purposes of replacement, such as when the teeth become worn or are broken. The stacks of blades may be inclined relative to a plane perpendicular to the axis of rotation of the hub; and the stacks may be alternated in this inclination or offset. That is, the blades of one stack may be inclined toward one end of the core and an adjacent stack inclined toward the opposite end of the core.
Prior to the present invention, all blades of whatever configuration and shape used on tire rasp hubs were of identical shape, size and distribution of teeth for that particular tire rasp hub. For instance, any two adjacent blades in a stack had identical secondary and tertiary configuration and the teeth of one blade were substantially laterally aligned with the teeth of the adjacent blade. This was usually facilitated by using identical blades throughout and fixing each blade of a stack in a xe2x80x9cname downxe2x80x9d direction, whereby the manufacturer""s name appeared on only one of the two opposed faces of the blade and indicated the direction in which the name side of all blades of that stack were to face. Where this technique was not employed, some other means of facilitating the stacking of the blades in a commonly aligned direction was used. Furthermore, all of the teeth on each blade were symmetrically disposed along the working edge of the blade. For instance, the tooth closest to one end was located the same distance from that end as the tooth closest to the other end was located from that other end. Thus, the blades and configuration of teeth remained the same even if the blade were reversed and the xe2x80x9cnamexe2x80x9d sides of adjacent blades faced each other.
I have found that by off-setting or indexing all of the teeth in alternate blades in a stack, a superior and longer lasting buffing performance and improved buffed texture are achieved, and the useful life of the blades is prolonged. The offset or staggered relation of adjacent teeth in a stack may be achieved by providing blades with a one-quarter pitch displacement that separates each tooth in the blade. The lateral offsetting may then conveniently and reliable be achieved by stacking the blades in an alternating xe2x80x9cname up,xe2x80x9d xe2x80x9cname downxe2x80x9d direction. The assembly of blades so formed presents one form of a staggered teeth array across each stack of the rasp hub.
The present invention thus provides a rasp blade having indexed teeth that can be used in the assembly of tire rasp hub having a staggered teeth array between adjacent blades in each stack of the rasp hub.
By using a tire rasp assembled with staggered teeth according to the invention, worn tire tread may be buffed away from a casing at a rapid rate while developing minimum amount of heat that might otherwise adversely affect the texture of the buffed surface needed for suitable retreading.
According to the present invention there is provided an arrangement of rasp blades on a tire rasp hub, wherein at least one stack of laterally spaced apart rasp blades is fitted to the rasp hub as an array extending circumferentially around the tire rasp hub (or slightly skewed), wherein adjacent rasp blades in a stack include a first blade having a first set of teeth uniformly distributed along a working edge of the fist blade at a predetermined pitch, and a second blade having a second set of teeth uniformly distributed along a working edge of the second blade. The second set of teeth has the same pitch as the teeth of the first blade; but they are located in a staggered relationship to the teeth of the first set. By xe2x80x9cstaggered,xe2x80x9d it is meant that by proceeding along a line perpendicular to the stacked blades and crossing a tooth in one blade, one will then cross the xe2x80x9ccut outxe2x80x9d or space between adjacent teeth on the next blade, followed by crossing a tooth, then a cut out, and so on, in this repeated pattern.
Preferable, the first blade has a first tooth of its set of teeth at a first predetermined distance relative to a reference end of its blade body, and the second blade has a first tooth of its set of teeth at a second predetermined distance relative to the end of its blade body closest to the reference end, the said first and second predetermined distances being offset by one-half of the pitch of the teeth to produce the a staggered relationship when the teeth are assembled in a stack.
In a preferred form, the first and second blades are of identical shape and configuration. Each of the said blades has a first tooth of its set of teeth at a first predetermined xe2x80x9clead inxe2x80x9d distance relative to a reference end of the blade and a second tooth of its set of teeth at a third predetermined or xe2x80x9clagxe2x80x9d distance relative to the end of the blade furthest from the reference end. By offsetting the lead in distance from the reference end by a quarter of the pitch so that the lag distance is three-quarters of the pitch, and then assembling the first and second blades adjacent one another and in oppositely facing directions (name up/name down) in the stack, adjacent teeth on adjacent blades are offset by one-half pitch.
Other feature and advantages of the present invention will be apparent to persons skilled in the art from the following description accompanied by the drawing wherein the same reference numerals will refer to like elements in the various views.